Desalting is the first process crude oil undergoes in a refinery. The primary purpose of desalting is to remove mineral salts present in crude oil, along with solids, metals, and water. Salts, mostly chlorides of sodium, potassium, and calcium naturally occur in soil and are associated with produced crude oil. Most of the salt is present as dissolved salt, in a small amount of water also associated with the crude oil. When this crude oil enters the refinery, it is necessary to remove these salts. Incomplete removal of salts can cause several problems, ranging from fouling and corrosion in heat exchangers and columns to catalyst poisoning.
Prior to entering the desalter vessel, crude oil is contacted with wash water by passing the two through a mixing valve. As a result, salt present in the crude oil is mixed with, and dissolved in the wash water. The mixing process also creates an emulsion of water drops in oil, which must be separated in order to remove the dissolved salt. This is accomplished in a desalter vessel, where the emulsion flows in at very slow velocities. An electric field in the desalter vessel promotes collision between drops, which leads to the formation of larger drops. When drops are sufficiently large, gravity forces the drops to settle to the bottom of the desalter vessel. Thus, the desalter vessel can be considered a gravity-based separation device, enhanced by the application of eclectic fields. In addition, chemicals may be added to promote drop coalescence.
With refineries increasingly processing more heavy crude oil, consistent desalting has become a challenge. The high density and viscosity of heavy crude oil, and its ability to form highly stable water-in-oil emulsions are mainly responsible for this inconsistent performance. Due to economic implications of poor desalting processes, it has become necessary to closely examine the fundamentals of the desalting process to investigate possible modifications. The fundamental process of water removal in a desalter vessel is the settling of water drops due to buoyant forces, which strongly depend on the size of the droplet. Thus, an important component of desalting is promoting drop-drop collisions, which result in the formation of larger drops.
Currently, optimization of desalter mixing is done primarily empirically, by tracing the performance of the desalter against mixing intensity usually controlled via the pressure drop across the mixing valve. Although there are some general principles on selecting the optimum pressure drop, there are no other controls used to alter mixing characteristics. This limits current desalting operation to the range of mixing produced by a single mixing valve.
Therefore, a need exists for a method and a system for increased coalescence rates in a desalter.